REPORT No. 272

. REPORT No. 272 THE RELATIVE PERFORMANCE OBTAINED WITH SEVERAL METHODS OF CONTROL . OF AN OVERCOIVH?RESSED ENGINE USING GASOLINE By ARTHUR TV. GARDIXER and W3EDON Langky Memorial AerctnauficaI Laboratory 327 . REPORT No. 2’72 THE RELATIVE PERK)RAI.AN!E OBTAINED WPMI SEVERAL METHODS OF CONTROL OF AN OVERCONIPRESSED ENGENE USING GASOLINE By ARTETR W. GARDIX-ER ancI WmLIAM E. W’H~DON SUMMARY This report presents some results obtained at the Langley Xemo~”al A.wonautical -lkbratory OJ the National Aiil%ory Cornmdtee for Aeronautics, during an investigation to determine the relatire perf ormanee characteristics for serei-al methods of control of an orercompressed engine using gasoline and operating under sea-lerel conditions. For this work, a special single cylinder test engine, /i-inch bore by Y-inch stroke, and designed for ready adjustment of compression ratio, valze timing am? cahe @ while running, was used. TAis engine has been fully described in N. A. ~- A. Technical Report No, 250. Tests were made at an engine speed of i ,400 B. P. X. for compression ratios ranging from .&O to T.6. The airduel ratios were on the rich side of the eherfiicaZZycorrect mixture and were approximately those giring maximum power. Men using plain domestic ariation gasoline, detonation was contro~ed to a constant, prede~ermined amount (audible), such as would be permissible for continuous operation, by (a) throttling the carbure~or, (6) maintaining fuZ throttle but greatly retarding the ignition, and (c) varying the timing of the inlet calce to reduce the e~eciire compre%- 5-ion ratio. For the first and third methods, the throttle opening and the oalce timing, respect irely, were adjusted so Llat tiie ignition timing cmdd be adranced slightly beyond the adcance gim”ng maxi- iizum power without ezceeding the standard of permissible detonation. The optimum performance for the engine when using a nondetonatingfuel, consisting of 80 per cent of commercial benzol and 20 per cent of aviation gasoline, was obtained as a ksisfor comparison. .— The following comparative results are based on the optimum performancefor the engine obtained with the nondetonating fuel at a compression ratio of .47. The poxer and f uel consumption with method (6) remained substantially constant at the higher compression ratios, the order of the ignition timing permitting full throttle operation ranging from SOOat 4.?’ to 3“ at 7.8; ezhaud temperatures, heat loss to the cooling water and explo~ion pressures at the higher ra~ios were normal. At a compression ratio of 7./7,the power obtained m“th method (a] was about 39 per cent lew and the fuel conwmption zoas considerably lover; uith method (b), time of inlet-rahe opening constant and &ime of in7et-Uake closing varied, the power was about 23 per cent less and t~e fuel co nsumpfion was greatly increased; with method (c), time of inlet opening and closing varied simultaneously, the power was about 29 per cent less and ~hefuel consumption was greatly increased. From these results, it may be concluded that method (b) gires the best all-round performance and, being easily employed in service, appears to he the most practicable ‘method for controlling an orercompressed engine using gasoline at 10w altitudes. INTRODUCTION Owing to the inherent advantages of higher specific output and reduced fuel consumption resulting therefrom, considerable attention has been directed toward increasing the expansion ratio of the earbureted engine, partiwlarIy the engines used in aircraft-, as it is in this field that the abo~e advantages are in greatest demand. However, coincidentally with an increase in the expansion ratio, there is, in conventional engines, a corresponding increase in the compression ratio wit h comeq uent aggravation of those conditions within the enatie inducing det om+ 9iXJ7-2&22 329 330 REPORJ? NATIONAL ADVISORY COMMITTEE FOR AEROX.4UTICS tion and preignition. For this reason, compression r@ios have been limited by the detonation characteristics of the fuels available for genera~ service use and, where higher compressions have been employed, it has been necessary to resort to the use of special fuels, which have no~ been, and are not now, at once cheap and generally available. Thus, the empkyment of higher compression ratios in aircraft= engines has not been generally adopted. However, as conditions inducing detonation hi the high-compression engine become less severe at high altitudes, a compromise has been sought, in which relatively high-compression ratios are employed in conjunction with throttling at low aItitudes to reduce the density of the charge and fihus suppress detonation until sufficient altitude has been gained to permit full-throttle operation, This scheme has received approval because it permits the use of fuels that are generally available, resuIts in increased fuel economy (greater expansiou ratio) at all altitudes, and gives an altitude range of constant power output. For operation below the altitude permitting full-throttle operation, the maximum permissible output; -eomparcd to the maximum obtainable with the same engine using a nondetonating fuel, is considerably reduced, and the specific weight of the engine, even when de~gned especially for the reduced stresses accompanying the reduced power, is increased. Thus, the questions arise: How much throttling is required to suppress detonation at low- altitud~s., does the performance under this condi- tion compare favorably with that of a no~mal engine., permitting full-throttIe operation \vitJl the common fuel} and is this method of control, by throttling, the most advantageous? In this connection, Ricardo has shown (Reference 1) that t.be specific output of an overcompressed engine throttled to prevent detonation is relatively low. But information showing the rela~ive performance obtained with other methods of controI is. apparently, not available. The present investigation was undertaken, therefore, at the Langley Memorial Aero- nautical Laboratory of the National Advisory Committee for Aeronautics iu connection with ,a research program invoking the determination of the &Qmparative flight performance of normal, overcompressed, and supercharged engines. The immediate object was to determine ~he relative performance of a single-cylinder, four-stroke-cycle, high-compression engine using domestic aviation gasoline with detonation controlled to a wnstant, predetermined, and permissible amount by throttling, retarding the ignition timing and \-arying the inlet valve timing. The full-throttle performance for optimum conditions when using a nondetonating fuel consisting of 80 per cent of commercial benzol and 20 per cent aviation gasoline was also obtained fc)r comparison, The range of compression ratios _used was from 4.0 to 7.6. The tests herein reported were conducted under.. the immediate. supervision of C!ljde R. Paton. DESCRIPTIO&_ For the present investigation a special, single~c.ylinder, four-stroke-cycle test engine, 5-inch bore and 7-inch stroke, connected to a 40/100 HP. electric cradle dynamometer was used. This engine, a transverse cross section of which is showu.in Figure 1, has been fully described in Reference 2. Briefly, the following adjustments can be made while the engine is running: A change in compression ratio from.4 to 14; a variatio~ in the time of opining and closing of both the inlet md exhaust vaIves of 50° measured in terms of crank tra-rel, the opening and cIosing being controlled independently, and a variation in the lift of both the inlet and exhaust valves from ~ inch tO M inch. Overhead val~~es are used, the two i-rdet and tht two eskmst. valves being operated by independent cam shafts. The water jackets for the cykder head and cylinder barre~ are separate, so that the temperature of the Cooling water for each may be controlled independently. For the present tests the compression ratio was varied from 4-to 7.5, and for all tests except those involving a varied timing the following valve adjustments were used: Inlet open . ____________________ JJl_degrees after top center. Inlet closed -------------------------- AL5degrees after bottom center. Inlet-valve lift ----------------------- .* inch. Exhaust open- -- -------------------- JiOdegrees before bottom center. Exhaust-closed ------------------------ M) degrees after top center. Exhaust-valve lift --------------------- .~ inch. METHODS OF CONTROL OF AN OTEP.C!OMPRESSED EXGIXE T7SIIN”G GASOLINE 331 FIG I.—Transrersesection throughsingIe-cyIiridertestengine,aho%ng adjustmentfor vrwing the compression ratiowhiIerunning 332 REPORT NATIONAL ADT71SORY COMMITTEE FOR AERONAUTICS For the tests with constami inlet-valve opening and Iate closing, the latter was varied from 45° to 108° after bottom center. For the tests with the total irdet phase displaced in the cycle, the phase was maintained at 125° and the time of opening varied from 10° to 60° after top center. Two porcelain-insulated spark plugs located diametrically opposite on the longitudinal axis of the cylinder were used; these were timed synchronously. A standard Liberty engine duplex carburetor was used, one Venturi being completely closed and the main metering jet for the remaining Venturi being provided with a needle -vaIve for ready adjustment of the fuel flow. Some tests were made with a small fixed metering jet, the needle ~,a]ve being omitted. FueI consumption was measured by timing the rate of flow from a 400 cm.3 tank Domestic aviation gasoline, conforming to Army spcwifications, was used as the detonating fuel. A mixture of 80 per cent of benzol (commercial 90 per cent) and 20 per cent a~iation gasoline (80–20 blend) was used as the nondetonating fueI. A diagrammatic layout of the induction system is shown on Figure 2. Air measurements were made with a gasometer having a displacement volume of 10 cubic feet; the gasometer belI 1 7ech-ical fiming Section x-x device showing boff[ing ; Wood frame, ~! T covered with ,. ba/[eon fabric, 1- ‘ ., Vo[. 74.6 CU. f+ “ ,: o-,, ,.. ,,,1 Corhurefor P A;r heafer,<beef mefal cylinde~‘ k=J’er VOfume 3.5 cu.

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